This invention relates to apparatus for the separating of mixtures in a wind tunnel.
In particular the apparatus relates to the separation of mixtures according to the specific gravity of the constituents of the mixture where the mixture has been graded according to size (i.e. sized) prior to the separation procedure. A particular application of the invention relates to the beneficiation of ores which may include gold ores, iron ore, coal, vermiculite, mica, metallurgical slag or other such material and tailings.
It has been proposed in the past to separate the constituents of a mixture having differing specific gravities by feeding the mixture into the top of a wind tunnel into a substantially stream line airstream through the tunnel and collecting different fractions of the materials in collectors arranged along the bottom of the tunnel, whereby constituents of lesser specific gravity are carried further along the wind tunnel than those of higher specific gravity.
One method of separation as disclosed in Australian Patent 520604 utilises a wind tunnel having an axial flow fan at the entry to the tunnel which provides the air flow through the tunnel. This arrangement, however, has been found to be cumbersome in that it requires a long transition section between the fan and the part of the tunnel in which the separation occurs in order to smooth out turbulence from the fan. The length of the transition zone is increased with an increased difference between cross-sectional area of the circular fan housing and the cross-sectional area of the square or rectangular wind tunnel. In such an instance, the air flow is not uniform across the wind tunnel even with a long transition zone.
An alternative proposal as disclosed in Australian Patent 545539 has been to utilise a centrifugal fan located at the entry to the wind tunnel which directs air transversely towards the entry. The air flow is subsequently redirected axially into the wind tunnel by means of deflectors and is streamlined by means of baffles. While the latter arrangement has the advantage of providing a wind tunnel of reduced axial dimensions in comparison to one utilising an axial fan, the use of deflectors to redirect the air flow from the fan results in difficulties in establishing a uniform laminar air flow under all flow conditions because the required deflector design varies with the air flow required to suspend material particles. This leads to losses in efficiency due to the need to establish increased pressure prior to the deflectors and baffles to overcome their resistance. The range of deflector designs required to accommodate the transportation of different particle sizes in parallel wind tunnel sections also creates difficulty in obtaining suitable fans to achieve optimum air volumes for each wind tunnel section.
Where tunnels according to each of the above proposals utilise air flow velocities ranging from 1.0 m/s to 7.0 m/s, it is possible (with some difficulty) to establish a reasonably streamlined flow through the tunnel by designing deflectors between the fan and tunnel. The design of the deflectors is based on trial and error since incremental changes in air flow do not involve corresponding incremental changes in shape or setting of the deflector elements. When the velocity of air is in excess of 7.0 m/s as is required for particle sizes above 3 mm, the air velocity is increased at the bottom of the plenum chamber in the region adjacent to the tunnel to such a degree that even with adjustable distribution plates there is non uniform air flow in the wind tunnel. Changes in plenum chamber geometry do not achieve uniform air flow for a range of air flows.
A further difficulty of each of the above proposals relates to the discharge of air which contains considerable amounts of residual inherent dust which can arise from the attrition of the material prior to screening or from the screening of the particulate material itself.
While each of the above prior art proposals has been found to operate satisfactorily under certain conditions, both have been found to be of a disadvantage where flexibility in use is required and where space is at a premium and portability is required. Furthermore, owing to transition zone requirements to produce uniform laminar air flows, the ability to increase the size of prior art separators is severely constrained.
A still further difficulty of each of the above proposals relates to the blinding of fine meshes, which are used to smooth out air flow, from insects, airborne particles and seeds which accumulate on the meshes thereby causing a restriction which lowers the air velocity relative to that selected.